This invention relates to tangentially fired, fossil fuel furnaces, and more specifically, to overfire air systems for reducing the NO.sub.x emissions from tangentially fired, pulverized coal furnaces.
Pulverized coal has been successfully burned in suspension in furnaces by tangential firing methods for a long time. The tangential firing technique involves introducing the fuel and air into a furnace from the four corners thereof so that the fuel and air are directed tangent to an imaginary circle in the center of the furnace. This type of firing has many advantages, among them being good mixing of the fuel and the air, stable flame conditions, and long residence time of the combustion gases in the furnaces.
Recently though, more and more emphasis has been placed on the minimization as much as possible of air pollution. To this end, most observers in the United States expect the U.S. Congress to enact comprehensive air emission reduction legislation by no later than the end of 1990. The major significance that such legislation will have is that it will be the first to mandate the retrofitting of NO.sub.x and SO.sub.x controls on existing fossil fuel fired units. Heretofore, prior laws have only dealt with the new construction of units.
With further reference in particular to the matter of NO.sub.x control, it is known that oxides of nitrogen are created during fossil fuel combustion by two separate mechanisms which have been identified to be thermal NO.sub.x and fuel NO.sub.x. Thermal NO.sub.x results from the thermal fixation of molecular nitrogen and oxygen in the combustion air. The rate of formation of thermal NO.sub.x is extremely sensitive to local flame temperature and somewhat less so to local concentration of oxygen. Virtually all thermal NO.sub.x is formed at the region of the flame which is at the highest temperature. The thermal NO.sub.x concentration is subsequently "frozen" at the level prevailing in the high temperature region by the thermal quenching of the combustion gases. The flue gas thermal NO.sub.x concentrations are, therefore, between the equilibrium level characteristic of the peak flame temperature and the equilibrium level at the flue gas temperature.
On the other hand, fuel NO.sub.x derives from the oxidation of organically bound nitrogen in certain fossil fuels such as coal and heavy oil. The formation rate of fuel NO.sub.x is strongly affected by the rate of mixing of the fuel and air stream in general, and by the local oxygen concentration in particular. However, the flue gas NO.sub.x concentration due to fuel nitrogen is typically only a fraction, e.g., 20 to 60 percent, of the level which would result from complete oxidation of all nitrogen in the fuel. From the preceding it should thus now be readily apparent that overall NO.sub.x formation is a function both of local oxygen levels and of peak flame temperatures.
Continuing, some changes have been proposed to be made in the standard tangential firing technique. These changes have been proposed primarily in the interest of achieving an even better reduction of emissions through the use thereof. One such change resulted in the arrangement that was the subject matter of U.S. patent application, Ser. No. 786,437, now abandoned, entitled "A Control System And Method For Operating A Tangentially Fired Pulverized Coal Furnace", which was filed on Oct. 11, 1985 and which was assigned to the same assignee as the present patent application. In accordance with the teachings of the aforesaid U.S. patent application, it was proposed to introduce pulverized coal and air tangentially into the furnace from a number of lower burner levels in one direction, and to introduce coal and air tangentially into the furnace from a number of upper burner levels in the opposite direction. As a consequence of utilizing this type of arrangement, it was alleged that better mixing of the fuel and air was accomplished, thus permitting the use of less excess air than with a normal tangentially fired furnace, which, as is well-known to those skilled in the art, is generally fired with 20-30% excess air. The reduction in excess air helps minimize the formation of NO.sub.x which, as noted previously herein, is a major air pollutant of coal-fired furnaces. It also results in increased efficiency of the unit. Although the firing technique to which the aforesaid U.S. patent application was directed reduces NO.sub.x, there were some disadvantages associated therewith. Namely, since the reverse rotation of the gases in the furnace cancel each other out, the gases flow in a more or less straight line through the upper portion of the furnace, thereby increasing the possibility of unburned carbon particles leaving the furnace due to reduced upper furnace turbulence and mixing. In addition, slag and unburned carbon deposits on the furnace walls can occur. These wall deposits reduce the efficiency of heat transfer to the water-cooled tubes lining the walls, increases the need for soot blowing, and reduces the life span of the tubes.
Another such change resulted in the arrangement that forms the subject matter of U.S. Pat. No. 4,715,301 entitled "Low Excess Air Tangential Firing System", which issued on Dec. 29, 1987 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 4,715,301, a furnace is provided in which pulverized coal is burned in suspension with good mixing of the coal and air, as in the case of the now abandoned U.S. patent application, which has been the subject of discussion hereinabove. Furthermore, all of the advantages previously associated with tangentially fired furnaces are obtained, by having a swirling, rotating fireball in the furnace. The walls are protected by a blanket of air, reducing slagging thereof. This is accomplished by introducing coal and primary air into the furnace tangentially at a first level, introducing auxiliary air in an amount at least twice that of the primary air into the furnace tangentially at a second level directly above the first level, but in a direction opposite to that of the primary air, with there being a plurality of such first and second levels, one above the other. As a result of the greater mass and velocity of the auxiliary air, the ultimate swirl within the furnace will be in the direction of the auxiliary air introduction. Because of this, the fuel, which is introduced in a direction counter to the swirl of the furnace, is forced after entering the unit, to change direction to that of the overall furnace gases. Tremendous turbulent mixing between the fuel and air is thus created in this process. This increased mixing reduces the need for high levels of excess air within the furnace. This increase mixing also results in enhanced carbon conversion which improves the unit's overall heat release rate while at the same time reducing upper furnace slagging and fouling. The auxiliary air is directed at a circle of larger diameter than that of the fuel, thus forming a layer of air adjacent the walls. In addition, overfire air, consisting essentially of all of the excess air supplied to the furnace, is introduced into the furnace at a level considerably above all of the primary and auxiliary air introduction levels, with the overfire air being directed tangentially to an imaginary circle, and in a direction opposite to that of the auxiliary air.
Yet another such change resulted in the arrangement for firing pulverized coal as a fuel with low NO.sub.x emissions that forms the subject matter of U.S. Pat. No. 4,669,398, entitled "Pulverized Fuel Firing Apparatus", and which issued on Jun. 2, 1987. In accordance with the teachings of U.S. Pat. No. 4,669,398, an apparatus is provided which is characterized by a first pulverized fuel injection compartment in which the combined amount of primary air and secondary air to be consumed is less than the theoretical amount of air required for the combustion of the pulverized fuel to be fed as mixed with the primary air to a furnace, by a second pulverized fuel injection compartment in which the combined primary and secondary air amount is substantially equal to, or, preferably, somewhat less than, the theoretical air for the fuel to be fed as mixed with the primary air, and by a supplementary air compartment for injecting supplementary air into the furnace, the three compartments being arranged close to one another. The gaseous mixtures of primary air and pulverized fuel injected by the first and second pulverized fuel injection compartments of the apparatus are mixed in such proportions as to reduce the NO.sub.x production. Moreover, the primary air-pulverized fuel mixture from the second pulverized fuel injection compartment, which alone can hardly be ignited stably, is allowed to coexist with the flame of the readily ignitable mixture from the first pulverized fuel injection compartment to ensure adequate ignition and combustion. An apparatus is thus allegedly provided for firing pulverized fuel with stable ignition and low NO.sub.x production.
Secondly, the apparatus in accordance with the teachings of U.S. Pat. No. 4,669,398 is characterized in that additional compartments for issuing an inert fluid are disposed, one for each, in spaces provided between the three compartments. The gaseous mixtures of primary air and pulverized fuel are thus kept from interfering with each other by a curtain of the inert fluid from one of the inert fluid injection compartments, and the production of NO.sub.x from the gaseous mixtures that are discharged from the first and second pulverized fuel injection compartments allegedly can be minimized. Also, the primary air-pulverized fuel mixture from the first pulverized fuel injection compartment and the supplementary air from the supplementary air compartment are prevented from interfering with each other by another curtain of the inert fluid from another compartment. This allegedly permits the primary air-pulverized fuel mixture to burn without any change in the mixing ratio, thus avoiding any increase in the NO.sub.x production.
Yet still another change resulted in the arrangement for firing pulverized coal as a fuel while at the same time effecting a reduction in NO.sub.x and SO.sub.x emission that forms the subject matter of U.S. Pat. No. 4,426,939, entitled "Method Of Reducing NO.sub.x and SO.sub.x Emission", which issued on Jan. 24, 1984 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 4,426,939, a furnace is fired with pulverized coal in a manner that reduces the peak temperature in the furnace while still maintaining good flame stability and complete combustion of the fuel. The manner in which this is accomplished is as follows. Pulverized coal is conveyed in an air stream towards the furnace. In the course of being so conveyed, the stream is separated into two portions, with one portion being a fuel rich portion and the other portion being a fuel lean portion. The fuel rich portion is introduced into the furnace in a first zone. Air is also introduced into the first zone in a quantity insufficient to support complete combustion of all of the fuel in the fuel rich portion. The fuel lean portion, on the other hand, is introduced into the furnace in a second zone. Also, air is introduced into the second zone in a quantity such that there is excess air over that required for combustion of all of the fuel within the furnace. Lastly, lime is introduced into the furnace simultaneously with the fuel so as to minimize the peak temperature within the furnace and so as to also minimize the formation of NO.sub.x and SO.sub.x in the combustion gases.
Although firing systems constructed in accordance with the teachings of the now abandoned U.S. patent application and the three issued U.S. patents to which reference has been made hereinbefore have been demonstrated to be operative for the purpose for which they have been designed, there has nevertheless been evidenced in the prior art a need for such firing systems to be improved. More specifically, a need has been evidenced in the prior art for a new and improved firing system that would be advantageously characterized by the fact that an advanced overfire air system is incorporated therein. To this end, the basic concept of overfire air has been proven to be the most cost effective method for controlling NO.sub.x in tangentially fired, fossil fuel furnaces. Overfire air is introduced into the furnace tangentially through additional air compartments, termed overfire air ports, that are designed as vertical extensions of the corner windboxes with which the tangentially fired, fossil fuel furnace is equipped.
The theory of NO.sub.x emissions reduction by overfire air is as follows. Operation with overfire air inhibits the rate of NO.sub.x formation by both atmospheric nitrogen fixation (thermal NO.sub.x) and fuel nitrogen oxidation (fuel NO.sub.x). The use of overfire air reduces the total oxygen available in the primary flame zone. As a result of this reduced oxygen zone, fuel nitrogen undergoes a recombination reaction to form molecular nitrogen, N.sub.2, rather than nitrogen oxide, simply due to insufficient oxygen in this zone and the intense competition with carbon species for the available oxygen. Consequently, the formation of NO.sub.x through fuel nitrogen conversion is greatly reduced. Similarly, overfire air operation results in reduction of thermal NO.sub.x formation through the temperature dependent Zeldovich mechanism. Heat release during the initial stages of combustion in the primary flame zone is somewhat reduced and delayed due to the reduced oxygen environment, with combustion ideally completed in the vicinity of the overfire air injection ports. The stretching of the heat release over a greater furnace volume results in lower peak combustion temperatures, thereby reducing thermal NO.sub.x formation.
Typical application of overfire air is through one or two closely grouped ports at a single fixed elevation at the top of the windbox, referred to as close-coupled overfire air, or at a higher elevation, referred to as separated overfire air. Experimental testing has shown a significant reduction in NO.sub.x with fossil fuel firing when, for a fixed total quantity of overfire air, the overfire air is introduced partly through close-couple overfire air ports and partly through separated overfire air ports. Moreover, experimental testing has shown that there exists a most favorable distribution of overfire air between the close coupled overfire air ports and the separated overfire air ports. In the case of bituminous coal, for example, this most favorable distribution has 1/3 of the overfire air flowing through the close coupled overfire air ports and 2/3 of the overfire air flowing through the separated overfire air ports.
In addition to the above, the manner in which overfire air is introduced into a furnace such that the air mixes with furnace gases in a controlled and thorough manner is also critical to maximizing overfire air effectiveness. Test data has shown that improvements in NO.sub.x emissions are attainable when the overfire air is injected from each furnace corner through two, three or more compartments with each compartment introducing a portion of the total overfire air flow at different firing angles such as to achieve a horizontal "spray" or "fan" distribution of air over the furnace plan area as compared to when other injection patterns are utilized for purposes of injecting the overfire air into the furnace. In addition, it has been found that through the use of such an injection pattern for the overfire air, furnace outlet conditions are also improved inasmuch as a more uniform flame pattern is created at the vertical outlet plane of the furnace. All tangentially fired, fossil fuel furnaces have a nonuniform flow pattern in the convective pass due to the tangential lower furnace flow pattern. This nonuniform flow pattern results in more flow on one side than the other and creates a side-to-side imbalance in steam temperature. The introduction of overfire air into the furnace by means of the injection pattern that has been described above wherein through the use thereof a horizontal "spray" or "fan" distribution of overfire air over the furnace plan area is had reduces this imbalance.
Finally, improved overfire air mixing with the furnace gases can be had by introducing the overfire air at high momentum. To achieve high overfire air momentum, the overfire air is introduced at velocities significantly above those typically employed in prior art firing systems, e.g., 200 to 300 ft./sec. versus 100 to 150 ft./sec. A boost fan may be needed to attain these higher overfire air velocities.
To thus summarize, a need has been evidenced in the prior art for such a new and improved firing system incorporating an advanced overfire air system that would be particularly suited for use in connection with tangentially fired, fossil fuel furnaces and that when so employed therein would render it possible to accomplish through the use thereof reductions in the level of NO.sub.x emissions to levels that are at least equivalent to, if not better than, that which is currently being contemplated as the standard for the United States in legislation which is being proposed. Moreover, such results would be achievable with such a new and improved firing system incorporating an advanced overfire air system without the necessity of requiring for the operation thereof any additions, catalysts or added premium fuel costs. Furthermore, such results would be obtainable with such a new and improved firing system incorporating an advanced overfire air system which is totally compatible with other emission reduction-type systems such as limestone injection systems, reburn systems and selective catalytic reduction (SCR) systems that one might seek to employ in order to accomplish additional emission reduction. Last but not least, such results would be attainable with such a new and improved firing system incorporating an advanced overfire air system which is equally suitable for use either in new applications or in retrofit applications.
It is, therefore, an object of the present invention to provide a new and improved advanced overfire air system for NO.sub.x control which is designed for use in a firing system of the type that is employed in fossil fuel-fired furnaces.
It is a further object of the present invention to provide such an advanced overfire air system for NO.sub.x control that is designed for use in a firing system of the type that is employed in tangentially fired, fossil fuel furnaces.
It is another object of the present invention to provide such an advanced overfire air system for NO.sub.x control that is designed for use in a firing system of the type employed in tangentially fired, fossil fuel furnaces such that through the use thereof NO.sub.x emissions are capable of being reduced to levels that are at least equivalent to, if not better than, that which is currently being contemplated as the standard for the United States in the legislation being proposed.
Another object of the present invention is to provide such an advanced overfire air system for NO.sub.x control that is designed for use in a firing system of the type employed in tangentially fired, fossil fuel furnaces characterized in that the advanced overfire air system involves the use of multi-elevations of overfire air compartments consisting of close coupled overfire air compartments and separated overfire air compartments.
A still another object of the present invention is to provide such a multi-elevation advanced overfire air system for NO.sub.x control that is designed for use in a firing system of the type employed in tangentially fired, fossil fuel furnaces and which is characterized in that there is a predetermined most favorable distribution of overfire air between the close coupled overfire air compartments and the separated overfire air compartments.
A further object of the present invention is to provide such an advanced overfire air system for NO.sub.x control that is designed for use in a firing system of the type employed in tangentially fired, fossil fuel furnaces and which is characterized in that the advanced overfire air system involves the use of a multi-angle injection pattern.
A still further object of the present invention is to provide such an advanced overfire air system for NO.sub.x control that is designed for use in a firing system of the type employed in tangentially fired, fossil fuel furnaces and which is characterized in that in accordance with the multi-angle injection pattern thereof a portion of the total overfire air flow is introduced at different firing angles such as to achieve a horizontal "spray" or "fan" distribution of overfire air over the plan area of the furnace.
Yet an object of the present invention is to provide such an advanced overfire air system for NO.sub.x control that is designed for use in a firing system of the type employed in tangentially fired, fossil fuel furnaces and which is characterized in that the advanced overfire air system involves the injection of overfire air into the furnace at velocities significantly higher than those utilized heretofore in prior art firing systems.
Yet a further object of the present invention is to provide such an advanced overfire air system for NO.sub.x control that is designed for use in a firing system of the type employed in tangentially fired, fossil fuel furnaces such that through the use thereof no additions, catalysts or added premium fuel costs are needed for the operation thereof.
Yet another object of the present invention is to provide such an advanced overfire air system for NO.sub.x control that is designed for use in a firing system of the type employed in tangentially fired, fossil fuel furnaces and which is characterized in that the advanced overfire air system is totally compatible with other emission reduction-type systems such as limestone injection systems, reburn systems and selective catalytic reduction (SCR) systems that one might seek to employ in order to accomplish additional emission reduction.
Yet still another object of the present invention is to provide such an advanced overfire air system for NO.sub.x control that is designed for use in a firing system of the type employed in tangentially fired, fossil fuel furnaces and which is characterized in that the advanced overfire air system is equally well suited for use either in new applications or in retrofit applications.